Water-in-oil-in-water (W/O/W) emulsions, which are multiphase emulsions having water-in-oil (W/O) droplets dispersed in their outer aqueous phase, are known as a formulation technique that can improve the percutaneous absorbability of active components as compared with other emulsion types (Non-patent Document 1), and are expected to find use in various applications including cosmetics, foods, and pharmaceuticals.
It is generally known that W/O/W emulsions are thermodynamically unstable systems so that when left standing for a long time, they are resolved or broken by creaming or flocculation followed by coalescence, whereupon they can no longer revert to the original stable emulsions.
In order to address this tendency, various attempts have been made to stabilize W/O/W emulsions. For example, the following methods have been known: a method for producing a W/O/W emulsion comprising an ascorbic acid derivative as an active component, in which a W/O emulsion having a water soluble active component dispersed in an inner aqueous phase is prepared using a dextrin fatty acid ester, and then dispersed and emulsified in an outer aqueous phase comprising an acrylic acid/alkyl methacrylate copolymer (Patent Document 1); and a method for producing a W/O/W emulsion, in which an electrolyte is added to an inner aqueous phase, the mixture is emulsified using a specified emulsifier to prepare a W/O emulsion, and the resulting emulsion is dispersed in an outer aqueous phase comprising an alkyl-modified carboxyvinyl polymer (Patent Document 2).
Meanwhile, since cosmetics are exposed to extremely harsh conditions in the stages of distribution and consumption among users, there has been a need to conduct stability investigations from more diverse perspectives, including change in properties of cosmetic products and chemical change in the active components to be incorporated. Further, the greatest interest among consumers is the usefulness of cosmetic products; thus, in the process of developing cosmetic products, the requirements for usability (e.g., impression from use, ease of use, preference) and functionality (e.g., moisture retention, skin lightening, prevention of skin roughness, anti-aging) need to be satisfied.